1. Field of the Invention
The present invention relates to photographic light-sensitive silver halide emulsions wherein the silver halide grains are spectrally sensitized to infrared radiation at wavelengths above 800 nm, to photographic elements and film units employing these emulsions and to a class of pentamethine cyanine dyes employed as the infrared sensitizing dyes.
2. Background Art
It is well known in the photographic art that the photosensitive response of silver halide emulsions can be extended to longer wavelengths by the addition of spectral sensitizing dyes, notably cyanine dyes. This technique has been employed to sensitize silver halide emulsions to a specific wavelength region in the visible and also the infrared portion of the electromagnetic spectrum and has been widely used in the production of photosensitive elements for color photography which comprise a plurality of spectrally sensitized emulsion layers that respond to different wavelength regions of the spectrum. This technique also has been employed in the production of panchromatically sensitized emulsions, generally by employing a combination of sensitizing dyes to provide the requisite sensitivity over the wavelength range of about 400 nm to 650 nm.
Various cyanine dyes have been used as spectral sensitizing dyes including polymethine dyes possessing a fluoro-substituted benzothiazole nucleus. For example, Kiprianov and Yagupolsky in J. Chem. USSR, 20, 211: Eng. Trans. 2187 (1950) disclose symmetrical and unsymmetrical cationic cyanine dyes obtained from derivatives of 6-fluorobenzothiazole. U.S. Pat. No. 4,387,155 is directed to certain unsymmetrical cyanine dyes useful as green sensitizing dyes which possess a benzoxazole nucleus and a 5-fluorobenzothiazole nucleus. U.S. Pat. No. 3,955,996 discloses a method for spectrally sensitizing a photographic light-sensitive emulsion employing a spectral sensitizing dye having an amidinium ion auxochrome and in columns 3 and 4 disclose numerous cyanine dyes including symmetrical and unsymmetrical polymethine dyes of fluoro-substituted benzothiazoles.